1. Field of the Invention
This invention relates to an apparatus and process for its operation which prevents the reoxidation of elemental mercury in gaseous matrices containing oxidizing compounds following the thermal reduction of oxidized mercury to elemental mercury. Such gaseous matrices are found, for example, in exhaust emissions of coal-fired boilers or incinerators. In particular, the apparatus, process and operation of the present invention may be employed in the analysis of total mercury in continuous emissions monitors used for regulatory compliance or process control measurements.
2. Description of the Prior Art
Various approaches in the development of continuous emissions monitors (CEMs) for mercury measurement in flue gas emissions to determine the elemental mercury therein, Hg(O), have included both a UV photometric method and an Atomic Absorption method. These methods detect and measure amounts of elemental or reduced mercury, Hg(O), at excitation wave lengths of 254 nm but unfortunately, any oxidized species of mercury therein cannot likewise be measured. In order to measure total mercury, the oxidized mercury species state must first be reduced to Hg(O). The predominant form of oxidized mercury existing in the flue gas coal fired combustors, waste combustors, and incinerators is HgCl.sub.2 C. S. Krivanek, III, Journal of Hazardous Materials, 47 (1996) Mercury Control Technologies for MWC's: The Unanswered Questions, pp. 119-136! and IEA Coal Research, Mercury Emissions and Effects--The Role of Coal!. By measuring the elemental mercury concentration in a gaseous matrix prior to and subsequent to a mercury reduction process, the total mercury concentration and the distribution of oxidized and reduced mercury can be determined. The determination of this distribution is essential to the development of effective mercury control options for gaseous emissions and can have value to effective process quality control. As used herein, the terms "elemental mercury," "metallic mercury," "reduced mercury" and "Hg(O)" all mean and refer to the same form of mercury.
Typically, in order to measure the total mercury concentration of a sample by laboratory analysis, the reduction of oxidized mercury involves the mixing of a gas or liquid sample with reducing solutions prior to measurement of Hg(0). In the development of on-line continuous emissions monitors (CEMs) for mercury measurement in the exhaust gasses of flue stacks, several instrument manufacturers have incorporated reducing solutions into their on-line processes. These devices rely on reducing solutions such as sodium hydroboride solution, stannous chloride solution or other reducing solutions to convert oxidized mercury to Hg(O) prior to measurement by a detector such as a ultraviolet (UV) or atomic absorption (AA) detector. An obvious disadvantage to this type of instrument design is that it requires frequent solution replenishment.
For continuous on-line measurement, a dry mercury reduction method is preferable to a wet one, since there are fewer maintenance requirements which, in turn, translates to a more reliable technique. Thermal reduction of oxidized mercury is such an alternative dry method, it being reported in the open literature that oxidized mercury can easily be reduced at temperatures of about 800.degree. C. However, a gaseous exhaust from a coal-fired boiler or incinerator may oftentimes contain oxidizing agents that can and will reoxidize the thermally reduced mercury before a mercury measurement can be effected.
The fossil-fueled and waste combustion industries generate gaseous mixtures which typically contain compounds such as NOx, O.sub.2, H.sub.2 O, CO.sub.2, and CO. Other gases such as SO.sub.2, HCl, Cl.sub.2, H.sub.2 S and NH.sub.3 and volatile metals and organics also may be present depending on the type of fuel combusted. Flue gases resulting from operation of coal combustors typically contain O.sub.2, CO.sub.2, CO, NO, NO.sub.2, SO.sub.2, HCl, N.sub.2 O, H.sub.2 O and mercury species as well as many trace components. Work leading to the instant invention determined that, of the components, supra, which are typically found in flue gas which have the greatest effect on the reoxidation of elemental mercury are, hydrochloride gas and oxygen. Furthermore, hydrochloride gas has the greatest effect on the reoxidation of mercury. Furthermore, it was found that the presence of oxygen in admixture with HCl gas acts to enhance the hydrochloride effect on elemental mercury oxidation.
The effect of hydrochloride on the reoxidation of elemental mercury upon thermal reduction is also reported by Wang, Xiao and Lindqvist, "Water, Air and Soil Pollution" 80: 1217-1226, 1995. They used crushed quartz chips to fill a quartz cell and thereafter heated their cell to 850.degree.-900.degree. C. to reduce Hg (II) in a gaseous stream. They also found the addition of HCl to the gaseous matrix negated the converter effect. Their approach to counter the HCl effect was to fill the converter with basic materials. Filling the quartz converter cell with a layer of soda lime or sodium carbonate or of crushed quartz treated with NaOH solution did improve the overall conversion efficiency by reacting the basic materials filling the conversion tube with hydrochloride (HCl) gas and preventing the reoxidation of elemental mercury. The effectiveness of this approach was limited however due to the severe corrosive nature of the basic solids and the high temperatures necessary for the conversion which they reported destroyed their converter cells within two days.
In view of the consideration and problems, supra, it should be appreciated by those skilled in this art that there is a definite need for a simple, reliable method to accurately measure and speciate mercury in the exhaust streams of flue stacks in real time at various waste and fossil-fueled combustors to measure mercury emissions and assist in identifying effective control technologies.